Passenger restraint systems are known which use an explosive pill to inflate an air bag, or to lock a seat belt or seat belt-shoulder belt combination system (see, for example U.S. Pat. No. 3,701,903, to which German patent disclosure document DE-OS No. 21 51 399 corresponds). In such a system, a piezoelectric crystal is utilized as a mechanical-electrical transducer, the mechanical-electrical transducer being connected to a voltage limiting device which, in turn, is connected to an integrator which then is connected to a threshold circuit. The threshold circuit, when the threshold is exceeded, provides the actual trigger pulses, for example, for an explosive pill, which, when triggered, permits compressed air to inflate an air bag, or to lock a belt restraint system. Upon impact, the mechanical-electrical transducer provides an output signal which, in case of a collision, is large enough to trigger the system to operation.
It has been found that shocks which should trigger the operation may not cause output signals from the integrator of such a nature that the threshold circuit is energized for a sufficiently long period to respond, so that the threshold circuit itself will provide an output signal only for a very short period of time, and the danger, then, may result that this extremely short output signal will be insufficient to reliably ensure that the explosive pill, or primer will be triggered. Consequently, reliable operation of the passenger restraint system is not ensured.
It has been proposed to construct the threshold circuit in the form of a Schmitt-trigger with hysteresis. This arrangement, however, does not influence the operation of the integrator and a certain minimum response period, required by the explosive charges from the threshold circuit is still not ensured.